Methods and systems for controlling flow of a diluted sample and determining pollutants based on water content in engine exhaust emissions

ABSTRACT

In a first embodiment of the present invention, a control method and system are provided in an exhaust emission sampling system for controlling a flow controller in order to compensate for the effects of changing water vapor content in a diluted sample having a predetermined dilution ratio. A water measuring device such as a relative humidity sensor generates a water vapor signal based on the amount of water vapor in the diluted sample. The water vapor signal is then processed within a control unit to obtain a value for the amount of water in the diluted sample. A control signal is generated by the control unit based on the amount of water vapor, flow rate of the exhaust emissions, and also, on the type of fuel used in the combustion process to control the flow controller which is, preferably, a mass flow controller.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to application entitled "Method andApparatus for Providing Diluent Gas to Exhaust Emission Analyzer" filedon Sep. 29, 1995 in the name of R. Neal Harvey et al., and having U.S.Ser. No. 08/536,401 now U.S. Pat. No. 5,756,360.

TECHNICAL FIELD

This invention relates to methods and systems for controlling flow of adiluted sample and determining pollutants based on water content inengine exhaust emissions and, in particular, to a method and system forcontrolling a flow controller and for determining final amount ofpollutants by compensating for the effects of changing water vaporcontent in engine exhaust emissions.

BACKGROUND ART

The constant volume sampler (i.e. CVS) was first applied in the late1950's to make possible the measurement of the mass of exhaustemissions. Before that time, emissions tests had been based onconcentration limits. Since the effect on the environment is assessed bythe grams of pollutants emitted by vehicles per mile driven, a samplingsystem was needed that could measure the mass of these emissions whilethe vehicle was operated through a sequence of accelerations anddecelerations that approximated normal driving.

The CVS and its operation are illustrated in FIG. 1 wherein vehicleexhaust enters the CVS at inlet 10, ambient air for dilution enters at12, and ambient bags and sample bags are indicated at 14 and 16,respectively. The CVS also includes a sample flow pump and a controller18, a measuring device 20 such as a CFV or SAO to measure CVS flow tocalculate volume, and a blower or pump 22 to set CVS flow rate.

All of the vehicle's exhaust is diluted with ambient air in therelatively large mixing "T" of FIG. 1. The combined gases are drawnthrough the system by the blower or pump 22 at a relatively constantcombined flow rate. Thus, a CVS operates at variable dilution ratio. Asthe vehicle produces more exhaust, less ambient air is mixed with it inorder to keep the total flow constant.

The measuring or metering device 20 in the bulk stream determines theflow rate. In this manner, the total volume of the mixture is easy todetermine from the time of the sampling multiplied by the constant flowrate. Today, CVS units actually do not quite operate at a constant rateof flow, but the name CVS is still used.

A small, proportional sample of the diluted gases is collected in thesample bags 16 during the sampling period. The sample bags 16 areanalyzed later for the concentrations of the pollutants. A simultaneoussample of the dilution air is also collected in the bag 14 forsubsequent analysis.

There are several types of CVS units available today, including fixedflow PDP and CFV types as well as variable flow SAO types.

The requirements of testing low emission vehicles make better samplingsystems necessary. Vehicles today may use alternative fuels such asMethanol, CNG and LPG. Burning these fuels produces more water,requiring more dilution, which lowers sample concentrations.

However, the same vehicles are likely to be LEVs and ULEVs, low andultra low emission vehicles. The emissions from these vehicles are verylow, and the concentrations have become very difficult to measureaccurately. It is clearly important not to over-dilute these samples,making them even more difficult to measure.

To meet these conflicting requirements, a CVS needs to operate at thecorrect flow rate for each vehicle, fuel and ambient condition. CVSunits based on fixed flow metering orifices however, are only able tooperate at a relatively small number of fixed flow rates.

At this point, the CVS method is very near its limit of capability foraccurate measurement of low amounts of pollution. The basic limitationarises from the method for diluting the exhaust gases. The only diluentavailable in sufficient quantity to be practical in a CVS is the ambientair in the test cell. However, this gas already contains considerablewater and has background concentrations of the pollutants that can be aslarge as the concentrations coming from the vehicle. So the CVS mustdilute the exhaust gases more than necessary with contaminated diluent.The sample concentrations that result are too low to be convenientlyanalyzed with conventional gas analyzers and the need to compensate foran almost equal amount of pollutant contributed by the backgrounddoubles the uncertainty of the measurement.

Mini-diluters are a new class of devices that avoid these limitations byreversing the order of the diluting and sampling of the exhaust gases,as shown in FIG. 2. The mini-diluter of FIG. 2 includes a pump 24 andsample bags 26.

As previously mentioned, while a CVS system must dilute all of thevehicle's exhaust at a variable dilution ratio, and then take aproportional sample, a mini-diluter reverses this process by firsttaking a small sample of the exhaust gas and then accurately diluting itat a relatively constant dilution ratio. Since only a small volume ofdiluent is needed, a dry, contaminant free gas, dry air or nitrogen, canbe used. Two advantages are the higher, more easily measuredconcentrations resulting from less dilution and the absence ofbackground contaminants, eliminating a need for a separate, error proneanalysis of the background pollution. The collection and analysis ofsamples of the diluent air is eliminated, doubling the accuracy of themass calculation.

A challenge for the mini-diluter is that the rate of collected sampleover a test period must be kept proportional to the raw exhaust flowfrom the vehicle, which is strongly varying, instead of the bulk streamflow through the CVS, which is relatively constant.

The following table illustrates the improvement in accuracy that can beexpected with a mini-diluter. It shows the expected sample bagconcentrations for a vehicle getting 25 mpg tested at 50% relativehumidity at 74° F. that meets the required ULEV emissions levels. Thetable compares the resulting bag concentrations for a standard CVS withfixed flow at 320 cfm, as referenced in the Code of Federal Regulations,a variable flow CVS, optimized for these conditions, and for amini-diluter.

    ______________________________________                                        Expected Bag Concentrations                                                   ULEV Limit   CVS        VFCVS      Mini-Diluter                               ______________________________________                                        HC    0.040  g/mi    1.1  ppm   1.9  ppm   3.3  ppm                           CO    1.70   g/mi    22.7 ppm   39.1 ppm   70.1 ppm                           NO.sub.x                                                                            0.20   g/mi    1.6  ppm   2.8  ppm   5.0  ppm                           ______________________________________                                    

It can be observed that the mini-diluter technique raises the bagconcentrations enough that it is feasible to measure these levels withthe same analyzer technology that is in use today.

All these sampling systems, fixed and variable flow CVS's as well asmini-diluter, must perform the following three functions:

Prevent condensation of water in the sample before it can be measured.The water content creates two problems. First, water condensing duringthe analysis process changes the concentration or the volume of thesampled gases, affecting the accuracy of the result. Second, somecontaminants, such as formaldehyde and NO_(x), are affected by theremoval of water.

Measure the total gas volume over a sampling interval, so the mass ofemissions can be calculated.

Collect a proportional sample of diluted exhaust in a sample bag foranalysis. At any time, the rate of flow of sampled gases and the totalrate of flow of diluted exhaust through the CVS must be in the sameproportion.

An improved mini-diluter of the above-noted patent application isillustrated in FIG. 3. The mini-diluter of FIG. 3 includes critical floworifices (CFO's) 28 to establish a stable dilution ratio. One of theorifices 28 is for the diluent, either nitrogen or dry air, and theother of the orifices 28 is smaller and is for the sample gas. Theorifices 28 operate on the principle from fluid dynamics that the flowthrough an orifice reaches a known maximum flow when the pressure dropacross it is large enough that the velocity in the orifice throatreaches the speed of sound. The two orifices 28 are appropriately sizedto provide a fixed dilution ratio appropriate for the type of fuel beingused.

A modified pressure regulator 30 of the mini-diluter maintains equalpressure at the inlets of the two orifices 28, even as the conditions atthe sampling point may change. A reference port is connected to thesample inlet and the action of the regulator 30 keeps the pressure atthe inlet of the diluent orifice equal to the pressure at the inlet tothe sample orifice.

The wet raw gases are brought to the dilution component via heatedlines, and the orifices 28 and pressure regulator 30 are kept in an oven32 to prevent any condensation of the sample before it is diluted. Theoven 32 also keeps the inlet temperatures of both orifices 28 at thesame temperature. Together with the action of the pressure regulator 30,this keeps the dilution ratio relatively constant.

The diluted gases can then be pumped by a pump 34 and then routed toeither a conventional analyzer bench 36 for a modal analysis or sent tosample bags 38. When used for modal sampling, the mini-diluter replacesthe usual sample conditioning unit, providing the advantages of muchless extracted sample and no modification of the sample by a cooler.

A mass flow controller 40 having a control valve (not shown) is used toproportion the flow to the bags 38. The absolute accuracy of the massflow controller 40 is not critical, only that its flow be kept inproportion to the vehicle exhaust flow. It is also important tocompensate for the gas transport delays from the sample point to theflow controller 40 so that the desired weighting of the collected gasesis correct.

As noted above, one approach to implementing a mini-diluter has been touse mass flow controllers, such as commonly used in the semiconductorindustry, to control the rates of flow of raw exhaust sample, diluentgas and proportional diluted sample. However, early attempts have notyet been entirely successfully demonstrated to be equivalent to the CVSmethod. The stability of the mass flow controllers and the sensitivityof their controlled flow rate to the composition of the flowing gases,as well as their slower response times to sudden changes in desiredflow, have presented difficulties.

One of the difficulties of sampling exhaust, which others may not fullyappreciate, is the changing water content of the sample. A keyrealization is that water is lost from the exhaust gas even inside thevehicle exhaust manifolds and piping, before it is even available to besampled, even by a mini-diluter using heated sampling lines. An engineis initially cold when testing begins and it is during this relativelybrief period that most of the pollutants are emitted. It is also duringthis period that the inaccuracies caused by lost water vapor fromcondensation in the exhaust manifolds and tailpipes have their greatestinfluence on the performance of the active elements of the mini-diluter.

The critical flow orifices 28 of FIG. 3 are affected by the changingwater content of the exhaust gases, but to a much lesser extent than arethe thermodynamic elements of a mass flow controller. The mass flowcontroller 40 of FIG. 3 is utilized only to proportion the sample to theexhaust flow. The absolute accuracy of the flow is not important, sincethis flow does not need to be measured.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and system forcontrolling a flow controller such as a mass flow controller in anexhaust emission sampling system based on changing water content of asample of the exhaust emissions.

Another object of the present invention is to provide a method andsystem for determining final amount of pollutants in an exhaust emissionanalyzing system based on changing water content of a sample of theexhaust emissions.

Still another object of the present invention is to provide a method andsystem for controlling flow of a diluted sample having a predetermineddilution rate in an exhaust emission sample system so that thepredetermined dilution ratio is held relatively constant despitechanging water content in the diluted sample.

In carrying out the above objects and other objects of the presentinvention in an exhaust emission sampling system including a flowcontroller for controlling the flow of a diluted sample of exhaustemissions from an engine's combustion process, the diluted sample havinga predetermined dilution ratio, a method is provided for controlling theflow controller in order to compensate for the effects of changing watervapor content in the diluted sample. The method includes the steps ofgenerating a water vapor signal based on the amount of water vapor inthe diluted sample and processing the water vapor signal to obtain avalue for the amount of water vapor in the diluted sample. The methodalso includes the step of generating a control signal based on the valueto control the flow controller which, in turn, controls the flow of thediluted sample.

Preferably, the step of generating the control signal is also based onthe flow rate of exhaust emissions and the type of fuel used in thecombustion process.

Also, preferably, the step of generating the water vapor signal includesthe step of sensing humidity in the flowing diluted sample.

Further in carrying out the above objects and other objects of thepresent invention in an exhaust emission sampling system including aflow controller for controlling the flow of a diluted sample of exhaustemissions from an engine's combustion process, the diluted sample havinga predetermined dilution ratio, a control system is provided forcontrolling the flow controller in order to compensate for the effectsof changing water vapor content in the exhaust emissions. The controlsystem includes a measuring device for generating a water vapor signalbased on the amount of water vapor in the diluted sample. The controlsystem also includes a control unit for processing the water vaporsignal to obtain a value for the amount of water vapor in the dilutedsample and for generating a control signal based on the value to controlthe flow controller which, in turn, controls the flow of the dilutedsample.

Preferably, the control unit generates the control signal also based onthe flow rate of the exhaust emissions and the type of fuel used in thecombustion process.

Still further in carrying out the above objects and other objects of thepresent invention in an exhaust emission analyzing system including anexhaust emission analyzer for analyzing a diluted sample of exhaustemissions from an engine's combustion process to obtain a preliminaryamount of pollutants, a method is provided for determining a finalamount of pollutants compensated for effects of changing water vaporcontent in the exhaust emissions. The method includes the steps ofgenerating a water vapor signal based on the amount of water vapor inthe diluted sample, processing the water vapor signal to obtain a valuefor the amount of water vapor in the diluted sample, and generating acorrection factor based on the value. The method also includes the stepof calculating the final amount of pollutants based on the correctionfactor and the preliminary amount of pollutants.

Preferably, the step of generating the water vapor signal includes thestep of sensing humidity in the flowing diluted sample.

Yet still further in carrying out the above objects and other objects ofthe present invention in an exhaust emission analyzing system includingan exhaust emission analyzer for analyzing a diluted sample to obtain apreliminary amount of pollutants, a determining system is provided fordetermining a final amount of pollutants compensated for the effects ofchanging water vapor content in the exhaust emissions. The determiningsystem includes a measuring device for generating a water vapor signalbased on the amount of water vapor in the diluted sample and a controlunit for processing the water vapor signal to obtain a value for theamount of water vapor in the diluted sample and for generating acorrection factor based on the value. The determining system alsoincludes a calculator for calculating the final amount of pollutantsbased on the correction factor and the preliminary amount of pollutants.

Preferably, the control unit also generates a control signal based on:(1) the value for the amount of water vapor in the diluted sample; (2)the flow rate of the exhaust emissions; and (3) the type of fuel used inthe combustion process.

The above objects and other objects, features, and advantages of thepresent invention are readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a prior art CVS;

FIG. 2 is a schematic block diagram of a prior art mini-diluter;

FIG. 3 is a schematic block diagram of another mini-diluter having amass flow controller;

FIG. 4 is a schematic block diagram of an exhaust emission samplingsystem which utilizes the control method and system of the presentinvention; and

FIG. 5 is a schematic block diagram of an exhaust emission analyzingsystem which utilizes the determining method and system of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to FIG. 4, there is illustrated with respect to themini-diluter of FIG. 3, a control method and system of the presentinvention wherein the components of FIG. 4, which are the same as thecomponents of FIG. 3, have the same reference numeral such as the pump34, the mass flow controller 40, and sample bags or sample bag unit 38.A CFO diluter unit 42 of FIG. 4 includes the pressure regulator 30, thediluent and sample CFO's 38 and the oven 32 of FIG. 3.

The control method and system of the present invention provides acontrolled portion of the flow of diluted sample to a water vapormeasuring device such as a relative humidity sensor or transducer 46which senses humidity and generates a corresponding water vapor signalbased on the amount of water vapor in the diluted sample. Typically,humidity variations are converted to capacitance variations of thesensor 46 in a well known fashion.

The water vapor signal is then received by a controller or control unit48 which is preferably a computer programmed to process the water vaporsignal to determine the amount of water vapor in the diluted sample andto generate a control signal based on the amount. The control signal isused to control the mass flow controller 40 and, in particular, tocontrol a control valve (not shown) within the controller 40 to vary themass flow of the diluted sample therethrough.

The control signal (i.e. Q_(sample) or n_(bag)) generated by the controlunit 48 is also based on flow rate of the exhaust emissions from thevehicle (i.e. Q_(Tp) or n_(ex)) and the type of fuel used in thecombustion process. This latter information is provided by a computer 50to the control unit 48.

The control unit 48 also calculates a correction factor (i.e. F_(corr))based on the value for the amount of water vapor in the diluted sample.The correction factor is then supplied by the control unit 48 to thecomputer 50 which, in turn, calculates a final amount of pollutants inthe exhaust emissions. This final amount is also based on a preliminaryamount of pollutants provided by analyzers 52 after the analyzers 52have analyzed the diluted sample for pollutants as is well known in theart.

Referring now to FIG. 5, there is illustrated a system for determiningthe final amount of pollutants in the exhaust emissions of an engine'scombustion process. The components of the system of FIG. 5 which are thesame as the components of FIG. 4 have the same reference numeral such asthe sensor 46, the control unit 48, the analyzers 52, and the computer50. Exhaust from an exhaust pipe 62 is fluidly coupled to an operatingengine (not shown) and by a tailpipe adapter 64 to an exhaust emissionsampler 66 such as the mini-diluter previously described. In general,the exhaust emission sampler 66 prepares a sample such as a dilutedsample of exhaust gas for analysis by the exhaust emission analyzers 52for analyzing the sample. The sampler 66 measures the concentration ofexhaust substances (i.e. emissions of, for example, CO, CO₂,hydrocarbons (HC), NO_(x), SO_(x), and the like) contained in theexhaust gas of an engine such as the engine of an automotive vehicle.

The analyzers 52 typically measure concentration of the exhaust gases inthe sample and provide corresponding exhaust gas concentration data to asystem bus 70, which may be a standard bus, which allows intersystemcommunication such as to the computer 50, a mass storage unit 74, and amonitor (not shown).

The computer 50 may be a PC having a sufficient amount of RAM and harddisk space for performing the algorithms associated with the presentinvention.

The system of the present invention may be programmed at the massstorage unit 24 to include a predetermined set of calculations asdescribed hereinbelow.

In general, and as described in mathematical terms hereinbelow, tocorrect for the effects of the changing water content in the dilutedsample, the control method and system of the present inventionpreferably uses a humidity sensor that measures the actual humidity ofthe sampled, diluted gases. A correction value in the form of a controlsignal is fed back to the mass flow controller 40 to compensate for theeffects of the changing water content on the dilution ratio that isestablished by the critical flow orifices.

Also, the determining method and system of the present invention alsouses humidity readings to determine a correction factor which, in turn,is utilized by the computer 50 to obtain final amounts of pollutantscompensated for changing water vapor.

Derivation of Mathematical Correction for Water Vapor

The object of a bag sampler and analytical system is to calculate thegrams of a pollutant that are emitted over a test interval. The actionof the diluting portion of the sampler is to dilute the sample by aknown factor K, so that: ##EQU1##

The action of the sampler that fills a sample bag at a flow rateproportional to the exhaust flow rate assures that:

    n.sub.bag (i.e. Q.sub.sample)=α·n.sub.ex (i.e. Q.sub.Tp)

The action of collecting the sample gases in a bag integrates thesequantities. At the end of a test phase or bag filling interval, theconcentration in the bag can be represented as: ##EQU2## Substitutingthe above relationships, this can be written: ##EQU3##

Ideally, K and α are held constant, so they can be taken out from underthe integral and we can write: ##EQU4## Which is the desired result: Thegrams from the test are obtained by multiplying the concentration in thebag by the total exhaust volume collected over the test phase, andapplying a constant dilution factor.

A difficulty arises because the factor K is not constant because thecomposition of the sample gas is changing as the amount of water in itschanges. A compensation can be made for this by adjusting the bag samplerate. If the dilution ratio increases, it can be compensated for byincreasing the sample flow rate into the bags. In other words, one makesthe proportionality constant α vary in time as K does. Specifically, oneprograms the control system so that: ##EQU5## Then the concentration inthe bag becomes: ##EQU6## This concentration differs from the desiredresult because of the α(t) term in the denominator. However, thecomputer controller of the mini-diluter device can easily calculate thisterm and calculate a correction factor of the form: ##EQU7## Theequation for the grams of pollutant becomes:

     P!.sub.bag ·V.sub.tp ·(1+K.sub.0)·F.sub.corr =grams.sub.p

This result provides a simple means to calculate the grams of pollutantfrom the concentration of a pollutant in a bag collected by themini-diluter.

Dilution Constant, K

The flow of a gas through an orifice is given by: ##EQU8## where Y isthe expansion factor and C₀ is the coefficient of discharge. Theexpansion factor depends on the specific heat ratio, k, of the gas. Thespecific heat ratio depends on the composition of the gas. ##EQU9##

When the pressure at the throat reaches a critical value, the speed ofthe flow in the throat reaches the speed of sound and the net flowcannot increase any further. The value for this critical pressure alsodepends on the specific heat ratio of the gas. ##EQU10##

In the mini-diluter of FIG. 4, exhaust sample flows through one of theorifices 38 and diluent gas, for example N₂, flows through the other.The sample pump 34 upstream of the orifices 38 provides sufficientvacuum so that both are operating at critical flow. The specializeddifferential regulator 30 assures that the inlet pressure to bothorifices 38 is the same, and the heated lines and cabinet insure thatthe temperatures at the orifices 38 are the same. Under theseconditions, the amount of dilution is determined by the properties ofthese gases and the size of the orifices 38.

From the continuity of the flow:

    Q.sub.md =Q.sub.N.sbsb.2 +Q.sub.samp

It is convenient to introduce K, the ratio of the diluent gas (i.e. N₂here) and sample flows, which is fixed by the orifice sizes and is abasic property of the mini-diluter. ##EQU11##

The value for K can be calculated from the orifice equation above. Usingthe ideal gas law and with appropriate cancellation of the common terms,this becomes: ##EQU12##

The above equations describe the theoretically expected value for K. Themini-diluter of FIG. 4 also includes provisions for a calibrationprocedure that determines K with a direct measurement. This compensatesfor any uncertainties in the orifice dimensions or dischargecoefficients. Plumbing is provided for calibration zero and span gasesto be conducted through the orifices 38. The ratio of the concentrationmeasured while calibration gas is conducted through both sample anddiluent orifices 38 to the concentration measured while calibration gasis conducted through the sample orifice and diluent is conducted throughthe diluent orifice provides an empirical measure of K. An adjustmentfactor then accounts for differences between the composition ofcalibration and exhaust sample gases.

The Variation of K

As noted above, the dilution constant K varies with time because theamount of water vapor in the exhaust varies with time. In the aboveequation for K, it is Y_(samp), M_(samp) and ΔP_(samp) that vary as thewater content of the sample changes. The changes in Y_(samp) andΔP_(samp) are due to small changes in the heat capacity ratio, k, andthe change in M_(samp) is due to the lighter density of water vapor. Theappropriate values to use at any moment in time can be calculated from aknowledge of the water content of the sample, which is provided by themeasurement device (i.e. sensor 46). The sensor 46, as previouslymentioned, is a relative humidity sensor, a dewpoint sensor or a waterconcentration analyzer. Any of these provide a measurement of the waterconcentration that is used in the following way to compute the M and kvalues.

The molecular weight of the sample is given by:

    M.sub.samp =18.0· H.sub.2 O!+44.0* CO.sub.2 !+28.0* N.sub.2 }

And the heat capacity ratio of the sample is given by the followingsequence of calculations: ##EQU13## where x, y and z are the atom countsin the molecular formula that represents the fuel. C_(p) and C_(v) arethe heat capacities of the gases at constant pressure and volume,respectively, and R is the ideal gas constant. The values 9.268, 8.011and 6.896 are the molar heat capacities of each species at the oventemperature.

The methods and systems of the present invention, as applied to themini-diluter design of FIG. 4, offer a significant improvement over theCVS sampling techniques. The mini-diluter of FIG. 4 uses simpler andmore reliable critical flow orifices 38 to fix the ratio of dilution ofthe sampled gases. The methods and systems of the present invention alsoapply to mass flow controllers, which are typically used where theirweaknesses do not affect the measurement. The methods and systems of thepresent invention correct for the varying water content of sampled gasesby measuring water content and feeding it back into the process.

While the best mode for carrying out the invention has been described indetail, those familiar with the art to which this invention relates willrecognize various alternative designs and embodiments for practicing theinvention as defined by the following claims.

What is claimed is:
 1. A method for controlling flow in an exhaustemission sampling system including a flow controller for controlling theflow of a diluted sample of exhaust emissions from an engine'scombustion process, the diluted sample having a predetermined dilutionratio, said method comprising controlling the flow controller in orderto compensate for effects of changing water vapor content in the dilutedsample, the method comprising the steps of:generating a water vaporsignal based on the amount of water vapor in the diluted sample;processing the water vapor signal to obtain a value for the amount ofwater vapor in the diluted sample; and generating a control signal basedon the value, which signal is forwarded to the flow controller which, inturn, controls the flow of the diluted sample.
 2. The method as claimedin claim 1 wherein the step of generating the control signal is alsobased on the flow rate of exhaust emissions from the engine.
 3. Themethod as claimed in claim 1 wherein the step of generating the controlsignal is also based on the type of fuel used in the combustion process.4. The method as claimed in claim 1 wherein the step of generating thewater vapor signal includes the step of sensing humidity in the flowingdiluted sample.
 5. In an exhaust emission sampling system including aflow controller for controlling the flow of a diluted sample of exhaustemissions from an engine's combustion process, the diluted sample havinga predetermined dilution ratio, the improvement comprising: a controlsystem for controlling the flow controller in order to compensate forthe effects of changing water vapor content in the exhaust emissions,the control system comprisinga measuring device for generating a watervapor signal based on the amount of water vapor in the diluted sample;and a control unit operatively connected to the measuring device and theflow controller for processing the water vapor signal to obtain a valuefor the amount of water vapor in the diluted sample and for generating acontrol signal based on the value to control the flow controller whichin turn controls the flow of the diluted sample.
 6. The sampling systemas claimed in claim 5 wherein the control unit generates the controlsignal also based on the flow rate of the exhaust emissions from theengine.
 7. The sampling system as claimed in claim 5 wherein the controlunit generates the control signal also based on the type of fuel used inthe combustion process.